Hey Red Mist, this critique is basically coming right out of "The Animator's Survival Kit". So, you should either get that book or look at it in a book store (page 220) if you want a superior explanation.
Basically, you should use the frames inbetween the extremes to make the animation exciting or interesting, and to convey extra information. Right now, your inbetween frames work, and look smooth, but they are just progressive interpolations of the two extremes. Each extreme in your animation is really cool, but the animation doesn't pop because whats happening in the middle is boring and predictable. Here I just made a few quick possibilities, to help explain. What you have inbetween really depends on what you're trying to achieve.
So here I just made a few crude possibilities. In the first one, the smile would kind of build with a closed mouth, then explode into a big open mouth smile. In the second, it may convey a little surprise or something, and the last, the face would be trying to contain the smile, fail and explode into a big smile.
Don't get me wrong, if you're not trying to convey something like that, then don't implement it. However, be aware of what you are conveying right now (seems to me like a child who is deliberately putting on a big cheesy smile for a laugh). Then, you can know how to express things, and make them more interesting when you need to. Overall I think the current piece is pretty neat, though.
